One o' t' three lines o' reference around which an aircraft can rotate. Avast, me proud beauty! T' three movements an aircraft can make (other than movin' straight ahead) all involve rotation around some central point. Avast! That point is called an axis. These movements are:
Roll: rotation around t' longitudinal (front-to-back) axis (nose remains pointed forward, but t' rocket spins),
Pitch: rotation around t' horizontal axis (nose moves up or down), and
Yaw: rotation around t' vertical axis (nose moves left or right.).